Some other proposed relics of the early universe, such as topological defects, can be copious emitters of neutrinos along with gamma rays and cosmic rays. Gamma rays and cosmic rays from these sources become severely depleted when propagating across the universe, while neutrinos reach Earth unimpeded. Finally, the detection of high-energy neutrinos from a known astrophysical source can be used to test the assumption of special relativity that photons and neutrinos have the same limiting speed, as well as the weak equivalence principle, according to which photons and neutrinos should suffer the same time delay as they pass through the gravitational potential of galaxies. Other departures from the Standard Model predictions, such as new physics at scales of beyond 1012 eV—the highest energies currently available from terrestrial accelerators—might also be inferred by studying the neutrino cross section on hadrons at energies well above 1012 eV.
IceCube is not the only option for a high-energy neutrino telescope. As mentioned, there are alternative technologies including the use of water (instead of ice) as the detector medium as well as techniques using radio or acoustic detectors still under development. As established above, a so-called gigaton detector is required to answer some of the key science questions. Will IceCube be unique in its abilities to address the questions? The jury is still out as to whether ice or water is a better detector and signal-transmission medium. (Detectors in ice suffer from scattering losses higher than those underwater, but ice is generally more transparent and possesses lower backgrounds, for example, from radioactive potassium-40 and bioluminescent marine life.) An expert panel of the International Union of Pure and Applied Physics (IUPAP) recently endorsed an underwater cubic-kilometer-scale follow-up to the NESTOR Mediterranean project, but no concrete proposals have been submitted as there is significant remaining research and development to determine the best design. As such, then, IceCube is unique in its stage of development, in its employment of ice as the detector medium, and in its location in the Southern Hemisphere.
The IceCube project involves scientists from institutions in the United States, Belgium, Germany, Japan, Sweden, the United Kingdom, and Venezuela and so in itself is an international effort. Plans call for the detector to be built in stages toward the full cubic-kilometer volume, over a 5- or 6-year period. Unlike many large-scale experiments, IceCube will be operational during the construction period. Currently, IceCube has a head start on its competitors, so its timely deployment will give it a lead in the exploration of this new window onto astrophysics.